Anti-HIV tests have transformed our understanding of the epidemiology of AIDS in the years since they were introduced in 1984, and they are still the bedrock of clinical diagnosis and much epidemiological research. Anti-HIV appears three weeks to three months after exposure to HIV and thereafter is invariably detectable in spite of any detrimental effect the virus may have on lymphocyte function and therefore antibody production. Neutralising antibodies to HIV are also measurable, but their titres are low. An inability to mount a neutralising response to HIV antigens together with the mutability of the virus are the most likely reasons why conventional approaches to preparing a vaccine have so far failed.
At first HIV antigen was prepared from infected cell lines. However, antigens can now be made by DNA cloning and expression or by synthesis of viral polypeptides. Several types of anti-HIV test exist, but most use a similar enzyme conjugate and give a colour signal due to the reaction between an enzyme specifically bound onto a polystyrene surface, membrane or inert particles and a substrate that then changes colour. Other tests depend on the binding of a fluorescein or chemiluminescent conjugate, or the visible agglutination of HIV-coated gelatin or latex particles.
Since anti-HIV tests became commercially available in 1985 they have been widely used in diagnostic and transfusion laboratories in the developed world. The accuracy — both sensitivity and specificity — of the antibody assays is continually being improved, and in competent hands the occurrence of false positive and false negative results is less and less frequent. The proportion of true to false positive results depends on the population studied, but even in low risk groups such as volunteer blood donors it is now very high in well conducted laboratories. Human, not test, errors cause most false results, and the key to avoiding these mistakes is continuous review with repeat testing where necessary. All positive reactions should both be confirmed by additional assays and succeeded by a test on a follow-up specimen (see below). The use of several screening tests in parallel on proven positive specimens also acts as a check on the possibility of false negativity in these assays (which it is otherwise difficult to guard against).
More discriminating tests can recognise the components of the antibody response. The serological response to individual HIV proteins can be studied by Western blot, and the immunoglobulin class response to HIV in blood and other fluids can also be investigated. The IgM response slightly proceeds the IgG response early in infection and is indicative of recent infection. Other test procedures, which employ both a highly sensitive and a "detuned" assay for anti-HIV are designed to detect infection within the previous few months and may therefore be used epidemiologically to measure incidence. The IgA anti-HIV response is a feature of infection in infancy.
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